Can someone explain Initial Timing, and Ignition Timing for me? i think i have a grasp on Mechanical & Vacuum advance.
Does it come from the Cam?
My Set up is on a sbc 350, 30 over. 186 double hump heads with 2.02/160 ported and polished, rpm performer manifold, Lunati VooDoo 268/276 cam, Holley 650 dp, 1 5/8 headers, New HEI dist, 350 turbo tran, 3.73 gears, B&M holeshot convertor. street truck that will be run hard... but not on the track. driven 30 miles to work (HWY) a couple of times a week.
Thanks!

Can someone explain Initial Timing, and Ignition Timing for me? i think i have a grasp on Mechanical & Vacuum advance.
Does it come from the Cam?
My Set up is on a sbc 350, 30 over. 186 double hump heads with 2.02/160 ported and polished, rpm performer manifold, Lunati VooDoo 268/276 cam, Holley 650 dp, 1 5/8 headers, New HEI dist, 350 turbo tran, 3.73 gears, B&M holeshot convertor. street truck that will be run hard... but not on the track. driven 30 miles to work (HWY) a couple of times a week.
Thanks!

Thanks to you all for the advive and two great reads... I'll have to read them a few more times and really look at them. One question keeps popping into my mind, does the 4 degrees of advance ground into the cam come into play while setting the initial timing? do you factor in that and set it, at say, 14 to 20dgr rather than the, 18 to 24dgr?

Also... I have a new HEI dist, do they sell Mech advance curve kits? I assume they are different weights and different tensions of springs, where according to my set up should I begin my testing with these kits or parts?

It will not provide a way to limit the mechanical advance. If you inspect the HEI, you may find that either bushings or another form of limiter method can (sometimes a simple set screw) be used to limit the travel of the rotor when the centrifical force acts upon it.

The kit does include the limit for the vacuum advance, also springs for the rate at which the mechanical advance comes in vs engine RPM. It also has an adjustable vacuum adv canister which can be used to fine tune the rate of vacuum advance vs engine vacuum.

Usually you shoot for a mechanical advance limiter that holds the total amount of mechanical advance to about 14-16 deg advance. But it all depends on the initial. Remeber the total between initial + mech should not exceed 36 Deg as a ball park. So if you set your initial at 18 then you can set your mech limit at 16 for a total of 34, if you set your initial at 22 then you need a mech limit of approx 12-14 deg.

Once you get that all straight then work on the vacuum advance limit and rate. Limit the vacuum advance at approx 10-14 deg max, this is critical, most cans add about 20-22 deg at a mere 8-10" wg vacuum, too much. the crane adjustable canister will allow you to tweak the amount of vacuum advance added vs engine vacuum IT DOES NOT LIMIT THE VACUUM ADVANCE, use the limiter plate included in the kit.

The absolute total between initial + mech + vacuum adv should not be over 50 Deg BTDC advance

The one word you used to describe getting it right is "testing",,,have fun

Ok... I think i'm getting it down little by little. I understand setting the intial timing with the timing light between 18-24 degrees.
How do you check the mech curve? with timing light? And, I see how a set screw can limit the mech advance.
The new dist has a canister that can be adjusted, with allen wrench, to adjust vacuum advance.
Having trouble understanding still how to set mech advance limit?
Testing the springs is easy...
The mech advance limiter is the plate that comes when the kit and looks easy to swap out. just curious how you check it? it's gotta be easy, but its got me stumped.

Ok... I think i'm getting it down little by little. I understand setting the intial timing with the timing light between 18-24 degrees.
How do you check the mech curve? with timing light? And, I see how a set screw can limit the mech advance.
The new dist has a canister that can be adjusted, with allen wrench, to adjust vacuum advance.
Having trouble understanding still how to set mech advance limit?
Testing the springs is easy...
The mech advance limiter is the plate that comes when the kit and looks easy to swap out. just curious how you check it? it's gotta be easy, but its got me stumped.

What distributor do you have? A stock GM HEI will use a different way to limit the mechanical advance than a MSD distributor, for example.

Unless you have a dial back timing light, the easiest way to set up the advance curve is to buy or make a timing tape or use a damper w/the outer ring indexed w/the degrees on it already.

Check the initial timing w/the vacuum advance disconnected and plugged. Note the reading. Then accelerate the engine until the mechanical advance quits rising- the amount you now see- minus the initial timing- is the amount of mechanical advance. In most cases you want the RPM where the mechanical advance quits advancing to be around 3000 rpm or less. This is set by changing the springs under the rotor that are connected to the weights of the mechanical advance.

The amount of vacuum advance needed is usually around 10-12 degrees. It can be set to come in/drop out by using the allen wrench adjustment. The idea here is to allow for as early (higher vacuum) of vacuum advance added as the engine will take w/o causing pinging on acceleration. It's a balancing deal and takes trial and error to get set right. The main thing is to not allow the engine to detonate (ping) under a load.

Good points cobalt, not sure if our OP is aware that the kit limiter plate has nothing to do with mechanical advance?

Quote:

Originally Posted by 60GR8Chevy

The mech advance limiter is the plate that comes when the kit and looks easy to swap out. just curious how you check it? it's gotta be easy, but its got me stumped.

The plate that comes with the kit is the vacuum advance limiter plate not the mech adv limiter. To install vacuum adv limiter plate on a HEI:

It goes in behind the canister actuation rod that is connected to the pickup ring on the distributor. Essentially it physically limits the travel of the rod which in turn limits the amount of vacuum adv timing added. Typically if you set it in there with the second last notch making contact with the actuation rod you will end up with approx 12 deg maximum vacuum advance added at full rod travel but it can vary with different distributors.

Note Installing the vacuum limiter plate or changing which notch is engaged will change the initial timing so that needs to be adjusted after installation or changing position of the plate. While changing the initial timing will not effect the vacuum advance characteristics we are talking about once they are set in.

Something else to consider,,,Each time you change which notch is engaged on the limiter plate you may need to re adjust the canister adjustment screw because the plate will shift the point at which the canister rod starts to move due to engine vac wg" applied to the canister diaphragm. Installing the plate on the back side of the actuator moves the range of the canister up. So if you think you have the can adjustment screw set were you want it and then add the plate better check your adjustment on the can again.

Once installed the limiter plate controls the maximum amount of vacuum advance added, the set screw in the can adjusts when the vacuum advance is added compared to engine vac.

I have never been able to adjust the crane canister for more than 12 in wg" as the top end of the range of the can, that is to say no matter how many times you turn the screw in the canister, the rod will always be at full travel once the vacuum level applied to the diaphragm is @ or more than 12 wg". This means that the timing added by the vacuum advance will be all in at engine vacuum levels at or above 12-13 wg". Do not turn the canister screw more than 9 or 10 turns out from all the way in, the screw will disengage from its internal threads and you have just pissed yourself off. It can be re-engaged with some effort.

A good place to start is 7 turns out (CCW) from all in. This will give a start point of about 6 wg" and an end point of approx 11 wg" with the plate in place. The approx 5 wg" range will remain the same and does not change much, the screw only shifts the zero (starting point).

Accell makes a can that is supposed to be able to limit the vacuum advance and also shift the range around without the plate limiter. I do not personally put faith in this canister because i have not tested it, one day I would like to bench test it, cut one open and see how this is done or if the claims are valid. The crane limiter plate works and the adjustable can does just that.

Put the plate in first then move to the initial timing setting, then work on the mechanical advance limit and curve as explained in other posts, then go back to the vacuum advance set screw tweaking. A useful method for me has been to get a vacuum gage with a long enough hose to reach the cab. Drive the truck and record what the engine vacuum level is at highway speed under light throttle load. If you are at say 12 "wg crusiing along at 70, try and get the vacuum advance canister rod to be fully engaged at 10 or 11 "wg using the screw. This can be done on the bench with a vacuum pump or just by lung power. The idea here is to have the vacuum advance start to drop out as soon as you give it more throttle on the highway but yet be fully engaged when cruising along.

This methodology and application is to provide a place to start, the proof is in the pudding.

If the engine rattles/pings no matter how you have the vacuum adv canister dialed in then you have more work to do on the initial and mechanical advance timing adjustments. Also gearing, trans shift points, type of fuel, compression ratio, air temperature, combustion chamber, piston type, A/F ratio and so on and so forth all effect the tendency of a motor to ping or not.

I have a dail back timing light, the harmonic balencer has numbers on it... still questioning how you check the Mech adv timing???
set the intial at 18 to 24 degrees, vacc disconected.
What next? keep reving up until timing stops advancing? is that it? Then subtract the intial timing and that gives me the Mech advance timing? Then install a set screw to prevent the weights from sling wide open, gring the screw head at small incerments until i get into an acceptable mech range?
Then finally buy the vaccum limiter plate to limit max vaccum?? keeping all numbers between 32 and 36 degrees total?
lastly adjust the vaccum canister on the dist with the allen wrench??? I read where F Bird says it's SO easy... this is kicking my ***!
I'm as confused as an infant in a titty bar!

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